**5.4 Preparation of the digester's bottom**

The pit's bottom should be cleaned, and the gridiron is built (Fig. 8) using a pre-selected type of iron rods as either 6Ø6 m-1 or 6Ø8 m-1. Subsequently, the concrete mixture is poured (Fig. 9 and 10). The water:cement ratio is 0.53 L kg-1 and the cement:sand:gravel mass ratio is 1:2.2:3.7. The thickness of the concrete base ranges between 10-25 cm depending on the soil's specifications and the ground water level.

Fig. 8. Structuring the grid irons for a commercial biogas plant (MT-ENERGIE GmbH & Co. KG)

Biogas Plant Constructions 355

In case of household units, burnt-clay bricks are used to build the digester (Fig. 12) and they should be able to tolerate a pressure up to 100 kg cm-2 owing to the fact that the walls of the digester are exposed to the pressures of the soil and the moving equipment near to the digester. A mortar of cement and sand mixture by 1:4 is used. The construction works are followed up till the appropriate height, and the entry or exit holes of the pipes are blocked

Fig. 11. Building the digester wall for a commercial biogas plant

Fig. 12. Building the digester wall of a household unit using burnt-clay bricks

Building the digester is associated with integrating the heating tubes. Building the wall starts with structuring the iron grids which will be encased by wood panels or preconstructed metal sheets, and before pouring the concrete, the heating tubes should be integrated (Figs. 11 and 13). The heating tubes are made of polyvinyl chloride (PVC), inside these tubes hot water flows to heat the digester. The water temperature is either 35 oC or 55 oC depending on the used bacteria as either mesophilic of thermophilic bacteria, respectively. On the other hand, in other designs the heating tubes are installed on the

by a filling material.

**5.6 Integrating the heating tubes** 

internal surface area of the digester wall.

Fig. 9. Pouring the concrete mixture for a commercial biogas plant (BIOGAS NORD GmbH)

Fig. 10. Concrete bottom of a household unit
